This application generally relates to induced nuclear reactions including processes, systems and elements, wherein a fuel component includes a means to release fission products therefrom during normal operation of a nuclear reactor and more particularly relates to a nuclear fission reactor, a vented nuclear fission fuel module, methods therefor and a vented nuclear fission fuel module system.
It is known that, in an operating nuclear fission reactor, neutrons of a known energy are absorbed by nuclides having a high atomic mass. The resulting compound nucleus separates into about 200 fission products (i.e., a residual nucleus formed in fission, including fission fragments and their decay daughters) that include two lower atomic mass fission fragments (i.e., a nucleus formed as a result of fission) and also decay products (a nuclide resulting from radioactive decay of a parent isotope or precursor nuclide). Nuclides known to undergo such fission by neutrons of all energies include uranium-233, uranium-235 and plutonium-239, which are fissile nuclides. For example, thermal neutrons having a kinetic energy of 0.0253 eV (electron volts) can be used to fission U-235 nuclei. Fission of thorium-232 and uranium-238, which are fertile nuclides, will not undergo induced fission, except with fast neutrons that have a kinetic energy of at least 1 MeV (million electron volts). The total kinetic energy released from each fission event is about 200 MeV for U-235 and about 210 MeV for Pu-239. In a commercial nuclear fission power reactor, this energy release is used to generate electricity.
During reactor operation, the aforementioned fission products may be released from a nuclear fuel pellet during the fission process. In the case of U-235 fission, typical fission products include isotopes of the elements of barium, iodine, cesium, krypton, strontium and xenon, among others. Some of these fission products are short-lived, such as I-131 which has a half-life of about eight days before beta decaying to Xe-131. Other fission products are longer-lived, such as Sr-90 which has a half-life of about 30 years. Production of solid and gaseous fission products or decay products thereof can affect operation of the nuclear reactor by having adverse effects on cladding material that house a plurality of the nuclear fuel pellets. These effects typically occur due to stress on the cladding because of increased internal pressure from the fission gases, contact of the fuel with the cladding due to swelling of the fuel (also known as fuel cladding mechanical interaction, FCMI), and chemical interactions of the myriad of fission products and existing or formed actinides with the cladding (also known as fuel cladding chemical interaction, FCCI). As an example of the former, fission product gases may accumulate in fuel rods containing the nuclear fuel and cause the fuel rod cladding to swell or deform plastically because of the increased internal pressure. As an example of FCMI, individual fuel pellets may swell volumetrically either across the entire fuel pellet or at the ends thereof to form an hour-glass shape. The mechanism leading to fuel pellet swelling that can compromise fuel cladding integrity is reasonably well understood by those in the art. In this regard, a gaseous fission product isotope may diffuse into the grain boundary of the fuel to form a gas bubble there, which leads, in part, to swelling of the fuel pellet. Additionally, solid phase fission products may precipitate out of the fuel matrix. Such processes contribute to the swelling of the fuel pellets. In either case, such swollen fuel pellets may bridge a heat transfer gap that is present between the fuel pellets and the cladding surrounding or housing the fuel pellets, thereby allowing the fuel pellets to contact the cladding. Contact of the fuel pellets with the cladding cause stress concentrations on the cladding as fission products continue to be formed leading to further fuel swelling. Fission products may migrate from the fuel pellet, travel into the heat transfer medium in the gap between the fuel pellet and cladding and may be either absorbed, adsorbed, or interact chemically with portions of the cladding, particularly at grain boundaries. In other words, the fission products, gaseous or otherwise, may accelerate stress corrosion cracking of the cladding, which may in turn lead to a breach of the cladding at the locally affected areas. It is understood that fission gas pressure, FCMI, and FCCI may interact upon the cladding in a manner such that the effects are compounded.
As previously mentioned, swelling of the fuel and build-up of fission gases can exert pressure on the fuel rod cladding that encloses the fuel material. Stresses, unless compensated for, might cause the fuel rod cladding to swell to the extent that coolant flow channels are obstructed. Also, such stresses, unless compensated for, might cause the fuel rod cladding to crack or rupture, as mentioned hereinabove. Thus, during the design phase of a nuclear fission reactor, reactor designers may shorten the design life of the nuclear fission reactor to compensate for the effects caused by accumulation of fission product solids and gases. Moreover, during operation of the nuclear fission reactor, reactor operators may be forced to temporarily shut-down the reactor to replace fuel rods that swell, crack or rupture due to effects of fission product gases.
There are various nuclear power reactor designs currently in use. Each of these designs produces fission products. For example, a pressurized water reactor (PWR) design, which uses thermal energy neutrons, includes a pressurizer that is partially filled with water. The water in the pressurizer is heated to create a steam bubble above the water that is in the pressurizer. The pressurizer, which is connected to a primary coolant loop of the reactor, provides an expansion space by means of the steam bubble to accommodate changes in water volume during reactor operation. Pressure is controlled in the primary coolant loop by increasing or decreasing the steam pressure in the pressurizer. Also, heat due to nuclear fission is transferred by conduction through the fuel cladding to water circulating in the primary coolant loop. Due to a relatively high pressure of about 138 bars (i.e., 2000 psi) in the primary coolant loop, coolant boiling is precluded in a PWR. A steam generator, that includes a secondary loop as well as the primary loop passing through it, is provided that allows the heat to transfer from the primary coolant loop to the secondary coolant loop. The secondary coolant loop is separate from the primary coolant loop, so that the coolant flowing through the secondary coolant loop is not radioactively contaminated by the radioactive coolant flowing through the primary coolant loop. Due to the heat transfer occurring in the steam generator, steam that is produced in the steam generator is eventually supplied to a turbine-generator for generating electricity in a manner well known in the art of electricity production from steam.
Moreover, fuel used in PWRs is typically uranium dioxide (UO2) sealed in a cladding made from a zirconium alloy, such as ZIRCALOY™ (trademark of the Westinghouse Electric Corporation, located in Pittsburgh, Pa., U.S.A.). For example, a specific cladding material that is in common use due to its low absorption cross-section for thermal neutrons and known resistance to corrosion and cracking is ZIRCALOY-2™, which contains chromium. A common composition given in the literature for ZIRCALOY-2™ contains about 98.25 weight % (wt %) zirconium (Zr), 0.10 wt % chromium (Cr), 1.45 wt % tin (Sn), 0.135 wt % iron (Fe), 0.055 wt % nickel (Ni) and 0.01 wt % hafnium (Hf). However, chemical interaction between the fission product cesium (Cs) and the chromium in the Zircaloy-2™ cladding may form the corrosion product compound cesium chromate (Cs2CrO4) that may conceivably attack the cladding. Other fission products, in addition to Cs, known possibly to attack ZIRCALOY-2™ include rubidium, cesium urinates, cesium zirconates, cesium halides, tellurium and other halogens, and fuel pellet impurities such as hydrogen, water and hydrocarbons. On the other hand, the cladding in a PWR may be made from materials other than ZIRCALOY-2™, such as ferritic martensitic steels. For example, Type AISI 304L stainless steel, which also contains chromium, has been used as another cladding material and contains C (0.02 wt %), Si (0.66 wt %), Mn (1.49 wt %), P (0.031 wt %), S (0.007 wt %), Cr (18.47 wt %), Ni (10.49 wt %) and Fe (68.83 wt %). Thus, the corrosion product cesium chromate may also be produced when stainless steel is used. However, it is known by persons of skill in the art of nuclear power reactor design that use of ZIRCALOY™, or ZIRCALOY-2™ or ferritic martensitic steels, even in the presence of fission product solids and gasses, reduces the risk of cladding corrosion, cracking or rupture to manageable levels for a given level of burn-up.
A boiling water reactor (BWR) design, which also uses thermal energy neutrons, allows coolant that acts as a moderator of neutrons to boil in the region of the fuel rods at a pressure of about 60 to about 70 bars (i.e., about 870 psi to about 1015 psi). This steam-water mixture is supplied to a water separator that separates the steam from the water. Thereafter the steam is supplied to a dryer that dries the steam. The “dried” steam is supplied to a turbine-generator for generating electricity in a manner well known in the art of electricity generation from steam. This reactor design does not use a secondary coolant loop or steam generator. In some cases, it may be desirable to remove fission products from the coolant, so that fission products do not contaminate the turbine-generator. The fuel in the fuel rods typically is UO2 and the cladding material typically is Zircaloy-2™. Thus, the pellet-clad interactions mentioned hereinabove for PWRs that might give rise to release of fission products may also obtain for BWRs. In addition, recirculation pumps may be used in BWRs to force recirculation of the coolant in order to control reactor power. The power history of the reactor in turn affects the amount and type of fission products produced.
A fast neutron reactor (FNR), such as a liquid metal fast breeder reactor (LMFBR) design, uses fast energy neutrons rather than thermal energy neutrons in the fission process. It is known that, in such fast neutron reactors, there is a greater excess of neutrons released during the fission process than in thermal neutron reactors. This excess of neutrons is used to breed fissile material through the absorption of the excess neutrons in fertile material. More specifically, the reactor core is surrounded by a blanket of non-fissile fuel materials, such as uranium-238, which is bred, or converted, to fissile fuel material, such as plutonium-239. The plutonium-239 can be reprocessed for use as nuclear fuel. It is known that such a method to operate and reprocess fuel within certain fast breeder reactors can lead to more fuel produced from the system than is consumed. The nuclear fuel present in the reactor core may be a uranium-nitride (UN). On the other hand, the fuel may be a mixed oxide fuel, such as plutonium dioxide (PuO2) and uranium dioxide (UO2). Alternatively, the fuel may be a metal actinide fuel produced by neutron capture during the fission process, such as an alloy of zirconium, uranium, plutonium and minor actinides (e.g., neptunium-237, americium-241, curium-242 through curium-248, berkelium-247, californium-249 through californium-252, einsteinium-252 and fermium-257). The reactor core is cooled by liquid metal, such as liquid sodium (Na) metal, or liquid lead metal, or a metal mixture, such as sodium-potassium (Na—K), or lead-bismuth (Pb—Bi). As is the case with all nuclear fission reactors, fission products are produced. Fission products absorb neutrons. Normally, in the breeder reactor fuel cycle, reprocessed fuel that is relatively free of neutron absorbing fission products is provided to the reactor core to generate heat that, in turn, is used to produce electricity. In this case, the fission products have been previously separated-out of the spent reactor fuel during reprocessing that occurs before the reprocessed fuel can be provided to the reactor core to produce the electricity. Therefore, it may be desirable to separate fission products from the fuel before reprocessing begins in order to more cost effectively reprocess the fuel.
An advanced gas-cooled nuclear fission reactor (AGR) uses a graphite neutron moderator and a carbon dioxide (CO2) coolant. AGRs obtain higher thermal efficiencies of about 40% and achieve higher burnups compared to PWRs and BWRs. The fuel is UO2 pellets clad in stainless steel. The coolant is circulated through the reactor core and then passed through a steam generator outside the core, but still within the pressure vessel. Reactor control of the fission process is by means of control rods and reactor shutdown is achieved by means of nitrogen injection into the reactor core. Injection of balls comprising boron provides a redundant shutdown capability. Fission product production may have similar effects on fuel rod integrity, as previously mentioned for PWRs, BWRs and FNRs. Fission products produced during operation of the AGR include technetium-99, ruthenium-106, cesium-134 and cerium-144, neptunium-237 and others.
There are other reactor designs under consideration in the nuclear industry but are, however, not in wide use. These other reactor designs include a light water cooled graphite-moderated reactor (coolant is boiling water); pressurized heavy water reactor (heavy water moderator, unenriched uranium fuel); sodium-cooled thermal reactor (thermal neutrons and sodium coolant); advanced pressurized water reactor (passive safety systems); simplified boiling water reactor (natural convection and no circulation pumps), among others. However, regardless of the reactor design, all nuclear fission reactors produce fission products that may have deleterious effects.
Thus, ameliorating the presence of fission product solids and gases in nuclear fuel rods for all reactor designs can help reduce risk of fuel rod swelling, cracking and rupture. Such amelioration may also reduce possible undesirable fission product gas and cladding chemical interaction which might lead to a breach of the cladding and release of fission products into the primary coolant system. Various systems are known in the art to prevent uncontrolled release of fission products into the primary coolant system. For example, fission products escaping into the reactor coolant may be scrubbed therefrom by use of filters and demineralizers.
A technique to remove fission gas from nuclear fuel is disclosed in U.S. Pat. No. 3,432,388, issued Mar. 11, 1969 in the name of Peter Fortescue and titled “Nuclear Reactor System With Fission Gas Removal.” This patent discloses a fluid-cooled nuclear reactor having a venting system for relieving pressure inside clad fuel pins. According to this patent, a passageway network interconnects the interiors of otherwise sealed clad fuel pins in different fuel elements, and gas is admitted thereto to initially bring the internal pressure to within a given increment of the coolant pressure at startup. When fission products cause the internal pressure to increase, gas is vented to storage vessels to maintain the internal pressure proportional to the coolant pressure.
Another technique to vent gaseous fission products is disclosed in U.S. Pat. No. 3,996,100 issued Dec. 7, 1976 in the names of Masaomi Oguma et al. and titled “Vented Nuclear Fuel Element.” This patent discloses a vented nuclear fuel element that comprises a cladding tube containing nuclear fuel therein and a device disposed in the upper portion of the cladding tube for venting gaseous fission products released from the nuclear fuel. The venting device comprises a porous plug for closure of the top end of the venting tube, which plug has a property of getting wet with the surrounding coolant, two plates that in cooperation with the cladding tube define a chamber for holdup of the gaseous fission products, a capillary tube for introducing the gaseous fission products from the nuclear fuel into the upper portion of the chamber, another capillary tube for introducing the gaseous fission products from the lower portion of the chamber to the porous plug, and a check valve for preventing the gaseous fission products within the chamber from flowing back into the interior of the cladding tube. Upon operation of the nuclear reactor, the gaseous fission products released from the nuclear fuel will pass through the check valve and the first mentioned capillary tube to reach the chamber, and from the chamber the gaseous fission products will pass through the second mentioned capillary tube and be vented through the porous plug to the coolant surrounding the nuclear fuel element.
The foregoing examples of related art and limitations associated therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.